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The biology of Megalopta genalis has been well-studied in central Panama. Two aspects of this bee have attracted special attention: its facultative expression of both solitary and eusocial behavior, and its nocturnal foraging.

Wicslo et al. (2004) describes the basic natural history of the species. M. genalis is a medium to large sweat bee, with head width ranging from 3 to 5 mm for females. Its head and thorax are metallic green, with coppery areas as well in the thorax. The abdomen is light brown. Like other nocturnal bees, the compound eyes and ocelli are relatively large for its size. M. genalis nests in dry, relatively soft, rotting sticks or lianas suspended above the forest floor. Nests are occasionally found on the ground, but they likely were initiated higher and fell down. Nests have been found several meters above the forest floor. The nest entrance is a circular entrance collar constructed of chewed wood that is just wide enough to allow entrance by the bees. The nest consists of a tunnel into the wood which is wider than the entrance collar. The tunnel may be as long as a meter or just a couple centimeters depending on the wood substrate. Likewise, the tunnel may be straight or branched depending on the wood. Most nests are 10 to 30 cm long. If the nest stick is wide enough to allow it, the bees may construct parallel tunnels. Cells are constructed linearly along the tunnel, and may be oriented parallel or perpendicular to the tunnel depending on the nature of the substrate. Brood cells are constructed so that the closed entrance of each cell is flush with the tunnel wall and forms part of the tunnel wall. Collected nests have contained as many as 19 cells, although most typically have 2-6. At least a few bees provision cells at all times of the year on Barro Colorado Island, Panama, where they have been most intensively studied, although very few have active brood cells during the latter part of the wet season (August-November). The bees are most reproductively active, and nests tend to have the most active brood cells, during the tropical dry season from December to April. Nearly all nests continue to have active brood cells through July, but not as many offspring as during the dry season. During the latter part of the wet season, most nests contain only adult females and the remains of old brood cells; ~5% contained active brood in a sample from August and September. The bees’ productivity corresponds to general floral availability: high in the dry season, less in the early wet season, and lowest from August-November.

M. genalis can nest either solitarily or in social groups with reproductive division of labor. The social biology of the species is described in detail by Wcislo et al. 2004; Smith et al. 2003, 2007, 2008, 2009; and Wcislo and Gonzalez 2006. As many as 11 females have been found in a nest, but studies of observation nests suggest that social groups are typically made up of a single queen and one, and rarely two, non-reproductive foragers. Additional females are newly emerged pre-dispersal reproductives who have not yet left the nest. Nests are initiated by a single foundress who constructs the tunnel and cells and provisions the first brood of offspring. In some nests, all offspring disperse, and the foundress continues provisioning more offspring. These are solitary nests, and the brood are usually predominantly, if not exclusively, male. In other nests, one, or sometimes two, females remain at the nest as non-reproductive helpers to their mother, who is now a social queen. The queen has enlarged ovaries, lays eggs, and rarely leaves the nest, while the workers have small ovaries, do not reproduce, forage for pollen, and feed the queen via trophallaxis (the mouth-to-mouth transfer of food) (Wcislo and Gonzalez 2006).

Smith et al. (2007) showed that social nests enjoy greater productivity than solitary nests, measured as new brood cells per week, although per capita productivity is equal between the two groups. Social nests also suffer less nest mortality. This is because nests fail when all of the adults present die. Even though the offspring are enclosed in cells with sufficient pollen and nectar for adult development, without parental defense at the nest, they fall prey to marauding ants (Smith et al. 2003). Nests are also occasionally attacked by vertebrate predators, likely anteaters and stick-probing birds, that consume all brood regardless of adults’ presence in the nest.

It is not clear how queen-worker caste differences are determined; this is an area of active investigation. However workers, that is, the foragers, are typically about 10% smaller than queens and also 10% smaller than their sisters who leave the natal nest to initiate new nests rather than stay and reproduce. This suggests that parental manipulation (because the mother controls the amount of nutrition available to the female offspring) and/or social dominance are involved in suppressing worker reproduction (Smith et al. 2008). Despite their smaller size, workers can become replacement queens. When the natural queen was experimentally removed from social nests, leaving the worker alone, the workers enlarged their ovaries and provisioned as many offspring brood cells as natural solitary nests active at the same time (Smith et al. 2009).

Natural enemies and associates: Ants are the primary predators of developing offspring in orphaned nests, although nests with adults present are well-protected (Smith et al. 2003). Nest sticks are occasionally discovered ripped open, slit open, or with holes accessing brood cells. This is presumably the work of anteaters or stick-probing birds, but an attack has never been witnessed.

Megalopta byroni is a cleptoparasite of M. genalis. It is not clear if this species is a cuckoo bee that deposits its eggs in M. genalis brood cells or if it takes control of social nests so that foragers provision its own offspring. In any case, it is rare, occurring in 0-3% of brood cells in a given collection (Biani and Wcislo 2007). The Riphorphoid beetle Macrosiagon gracilis (Falin et al. 2000) and the mutilid Lophostigma cincta (Cambra et al. 2005) are also cleptoparasites of M. genalis that kill and consume M. genalis pre-pupae or pupae. Together, these three parasites infect less, often much less, than 5% of brood cells in a given collection, although M. gracilis seems to be more common in the dry season. The Chlorpid fly Fiebrigella sp. is also a cleptoparasite of M. genalis, but it does not kill the developing offspring. Rather, it consumes some of the pollen intended for the bee. An infected brood cell may contain as many as 20 flies, but 1-6 is more typical. Fly parasitism of the pollen causes the developing offspring to be smaller than it otherwise would have been, which may affect the social caste of the parasitized bee, although this requires further research. Cells are more often infected in the wet than dry season (Smith et al. 2008).

Adult M. genalis are attacked by a parasitoid conopid fly which develops inside the thorax, killing the bee.

Many M. genalis harbor nematodes of the species Acrostichus megaloptae in their Dufour’s gland (Kanzaki et al. 2010). This gland produces the hydrophobic coating that the bees use to line their brood cells, but infected individuals can nest successfully. It is unclear what, if any, effect these nematodes have on the bees. Likewise, many bees carry individuals of the phoretic mite Histiostoma halictonida with unknown effects (Engel and Fain 2003).

Lastly, the mite Laelaspoides sp. is often found in the brood cells of M. genalis, including on the developing offspring. It is mutualistic with the bees because it reduces fungal infection in the brood cells (Biani et al. 2009).

Nocturnal foraging: Unlike most bees, but like all of the other non-parasitic species of Megalopta, M. genalis fly during the approximately 70 minutes before sunrise and after sunset, during astronomical twilight (Kelber et al. 2006). They remain in the nest during the day and during the middle of the night, and they may not leave for every foraging period, or only for a short flight during a given foraging period. M. genalis are apparently flying at the very limits of their visual ability, because bees returning to the nest early in the morning or late at night (that is, during the darkest times that they forage) make more mistakes and have to take slower, less direct flight paths when returning to their nest stick (Theobald et al. 2007). To human eyes, the forest understory is pitch dark at these times. In addition to large compound eyes and ocelli, M. genalis use lateral neural connections between the optic nerves coming from each facet of the compound eye (neural summation) to increase their light detection ability (at the cost of lower spatial resolution) (Warrant et al. 2004). Because M. genalis fly at night, they forage from nocturnal flowers or diurnal flowers that open before dawn. On Barro Colorado Island, where their diet has been studied, they use at least 65 species of pollen in the dry and early wet seasons. However, their diet in the dry season is dominated by trees in the Malvaceae subfamily Bombacoidae, especially Pseudobombax septenatum. In the wet season Hura crepitans (Euphorbiaceae) is their most common food source. These trees are apparently adapted to bat pollination, but it is unknown whether M. genalis may also serve as a pollinator.


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© Smith, Adam

Source: Halictidae

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